Daily variability of dissolved inorganic radiocarbon at three sites in the surface ocean

We report radiocarbon measurements of dissolved inorganic carbon (DIC) in surface water samples collected daily during cruises to the central North Pacific, the Sargasso Sea and the Southern Ocean. The ranges of Δ¹⁴C measurements for each cruise (11–30‰) were larger than the total uncertainty (7.8‰, 2-sigma) of the measurements. The variability is attributed to changes in the upper water mass that took place at each site over a two to four week period. These results indicate that variability of surface Δ¹⁴C values is larger than the analytical precision, because of patchiness that exists in the DIC Δ¹⁴C signature of the surface ocean. This additional variability can affect estimates of geochemical parameters such as the air–sea CO₂ exchange rate using radiocarbon.     

The feasibility of isolation and detection of fullerenes and carbon nanotubes using the benzene polycarboxylic acid method

The incorporation of fullerenes and carbon nanotubes into electronic, optical and consumer products will inevitably lead to the presence of these anthropogenic compounds in the environment. To date, there have been few studies isolating these materials from environmental matrices. Here we report a method commonly used to quantify black carbon (BC) in soils, the benzene polycarboxylic acid (BPCA) method, for measurement of two types of single walled carbon nanotubes (SWCNTs), two types of fullerenes and two forms of soot. The distribution of BC products (BPCAs) from the high pressure and high temperature oxidation illustrates the condensed nature of these compounds because they form predominantly fully substituted mellitic acid (B6CA). The conversion of carbon nanoparticles to BPCAs was highest for fullerenes (average of 23.2 ± 4.0% C recovered for both C₆₀ and C₇₀) and lowest for non-functionalized SWCNTs (0.5 ± 0.1% C). The recovery of SWCNTs was 10 times higher when processed through a cation-exchange column, indicating the presence of metals in SWCNTs compromises the oxidation chemistry. While mixtures of SWCNTs, soot and sediment revealed small losses of black carbon during sample processing, the method is suitable for quantifying total BC. The BPCA distribution of mixtures did not agree with theoretical mixtures using model polyaromatic hydrocarbons, suggesting the presence of a matrix effect. Future work is required to quantify different types of black carbon within the same sample.     

Constraining the propagation of bomb-radiocarbon through the dissolved organic carbon (DOC) pool in the northeast Pacific Ocean

This study extends the 1991-1995 records of marine dissolved organic carbon (DOC) concentrations and Δ¹⁴C values at hydrographic Station M (34°50′N, 123°00′W) with new measurements from a frozen (-20 °C) archive of samples collected between April 1998 and October 2004. The magnitudes and synchronicity of major Δ¹⁴C anomalies throughout the time-series imply transport of DOC from the surface ocean to depths of at least 450 m on the timescale of months. Keeling plots of all measurements at Station M predict a continuum of possible background DOC compositions containing at least 21 μM of -1000‰ (i.e., ≥57,000 ¹⁴C years) DOC, but are more consistent with mean deep DOC (38 μM, -549‰; i.e., 6,400 ¹⁴C years). These results and coral records of surface dissolved inorganic carbon (DIC) Δ¹⁴C were used to estimate pre-bomb DOC Δ¹⁴C depth profiles. The combined results indicate that bomb-¹⁴C has penetrated the DOC pool to depths of ≥450 m, though the signal at that depth is obscured by short-term variability.     

Source(s) and cycling of the nonhydrolyzable organic fraction of oceanic particles

A major fraction of particulate organic carbon (POC) in the deep ocean remains molecularly uncharacterized. In an effort to determine the chemical characteristics and source(s) of sinking POC, we studied a nonhydrolyzable fraction of sinking POC using ¹³C NMR (nuclear magnetic resonance) spectroscopy and analytical pyrolysis. ¹³C NMR spectra and products from analytical pyrolysis of the nonhydrolyzable fraction exhibit a strongly aliphatic character that is distinct from that of bulk POC. The aliphatic nature of this fraction is consistent with its low stable carbon isotope values. We hypothesize that the nonhydrolyzable fraction derives to a significant extent from a refractory component of organisms that selectively accumulates, resulting in its manifestation as a major part of POC sinking to the deep ocean and in underlying sediments.     

CLIMATE HISTORY INFERRED FROM BOREHOLE TEMPERATURES, DATA FROM THE CZECH REPUBLIC

The knowledge of the present-day underground temperatures may be important in the assessments of the past climate change. The method of inversion of the temperature-depth records into the ground surface temperature history is briefly introduced by showing an example of synthetic data and illustrated by a review of existing results obtained from the inversion of temperature logs measured in holes in the Czech Republic. Underground temperatures observed in holes of the depth of at least 1000–1500 m seem to confirm the preinstrumental climate pattern of the past several thousand years. Most of shallower temperature records (500–800 m) revealed general warming of climate followed the Little Ice Age of the 17–18th centuries and a pronounced increase of the soil temperatures by at least 1 K since the beginning of this century.     

Changed late Quaternary marine environments on Atlantic continental shelf and upper slope

About 2000 large sediment samples were collected during the early 1960s throughout the continental shelf off the Atlantic coast of the United States to establish and map sediment types including sediments relict from times of glacially low (and subsequently higher) sea levels. In about 510 of these samples we found fossil shells of mollusks remaining from environmental conditions different from those at present. Publications and collections by others contain about 70 additional samples having relict mollusks. Some of these shells indicate lower sea levels, others colder water, and still others warmer water than is now present. Radiocarbon measurements from earlier studies by us and others established the dates of colder water (late Pleistocene), and we made additional measurements to learn the dates of warmer water (about 1000 to 2000 yr B.P.). The results show reasonably enough that continental shelves are the sites of relict faunas as well as of sediments that indicate changed and complex environmental histories.     

Incorporation of aged dissolved organic carbon (DOC) by oceanic particulate organic carbon (POC): An experimental approach using natural carbon isotopes

Incorporation of ¹⁴C-depleted (old) dissolved organic carbon (DOC) on/into particulate organic carbon (POC) has been suggested as a possible mechanism to explain the low Δ¹⁴C-POC values observed in the deep ocean [Druffel, E.R.M., Williams, P.M., 1990. Identification of a deep marine source of particulate organic carbon using bomb ¹⁴C. Nature, 347, 172–174.]. A shipboard incubation experiment was performed in the Sargasso Sea to test this hypothesis. Finely ground dried plankton was incubated in seawater samples from the deep Sargasso Sea, both with and without a biological poison (HgCl₂). Changes in parameters such as biochemical composition and carbon isotopic signatures of bulk POC and its organic compound classes were examined to study the roles of sorptive processes and biotic activity on POC character. Following a 13-day incubation, the relative abundance of the acid-insoluble organic fraction increased. Abundances of extractable lipids and total hydrolyzable amino acids decreased for both treatments, but by a greater extent in the non-poisoned treatment. The Δ¹⁴C values of POC recovered from the non-poisoned treatment were significantly lower than the value of the unaltered plankton material used for the incubation, indicating incorporation of ¹⁴C-depleted carbon, most likely DOC. The old carbon was present only in the lipid and acid-insoluble fractions. These results are consistent with previous findings of old carbon dominating the same organic fractions of sinking POC from the deep Northeast Pacific [Hwang, J., Druffel, E.R.M., 2003. Lipid-like material as the source of the uncharacterized organic carbon in the ocean? Science, 299, 881–884.]. However, the Δ¹⁴C values of POC recovered from the poisoned treatment did not change as much as those from the non-poisoned treatment suggesting that biological processes were involved in the incorporation of DOC on/into POC.     

High resolution isotope study of the latest deglaciation based on Bermuda Rise cores

A stable isotope and ¹⁴C investigation of carbonates from three late Quaternary cores with high rates of sedimentation from the northeastern Bermuda Rise has produced the highest resolution record of the glacial maximum to Holocene stable isotope change yet obtained from an open ocean location. The record includes a three-step “termination” and the first direct evidence of an early deglacial meltwater “spike” in the open sea.     

Large-volume ultrafiltration for the study of radiocarbon signatures and size vs. age relationships in marine dissolved organic matter

In recent decades, tangential-flow ultrafiltration (UF) technology has become a primary tool for isolating large amounts of “ultrafiltered” marine dissolved organic carbon (UDOC; 0.1 μm to ∼1 nm) for the detailed characterization of DOC chemical composition and radiocarbon (Δ¹⁴C) signatures. However, while total DOC Δ¹⁴C values are generally thought to be quite similar in the world ocean, previous studies have reported widely different Δ¹⁴C values for UDOC, even from very similar ocean regions, raising questions about the relative “reactivity” of high molecular weight (HMW) DOC. Specifically, to what degree do variations in DOM molecular weight (MW) vs. composition alter its relative persistence, and therefore HMW DOC Δ¹⁴C values?In this study we evaluate the effects of varying proportions of HMW vs. low molecular weight (LMW) DOC on UDOC Δ¹⁴C values. Using concentration factor (CF) as a proxy for MW distributions, we modeled the retention of both OC and Δ¹⁴C in several very large CF experiments (CF >3000), from three depths (20, 670, and 915 m) in the North Pacific Subtropical Gyre (NPSG). The resulting DOC and Δ¹⁴C UF permeation coefficients generally increase with depth, consistent with mass balance trends, indicating very significant permeation of LMW, ¹⁴C-depleted DOC at depth, and higher recoveries of Δ¹⁴C-enriched, HMW DOC in the surface. In addition, changes in CF during sample concentration and ionic strength during sample diafiltration had very large and predictable impacts on UDOC Δ¹⁴C values.Together these results suggest that previously reported disparities in UDOC Δ¹⁴C values are reconciled by linked trends of Δ¹⁴C content vs. MW. At low CFs, UDOC samples have similar Δ¹⁴C values to total DOC. In contrast, UDOC samples collected at extremely high CFs (and after diafiltration) have more positive Δ¹⁴C values. We demonstrate that the observed relationships between UDOC Δ¹⁴C and CF derived from our data can directly explain offsets in all previously published UDOC Δ¹⁴C values for the NPSG. While CF is not traditionally considered in UF studies, our results indicate it can substantially influence the interpretation of UDOC ¹⁴C “age”, and thus reactivity, in the marine environment. In addition, our results indicate that CF can in fact be used as a proxy for average MW. We suggest that a variable-CF-UF approach, coupled with molecular-level Δ¹⁴C analyses, presents a new tool for studying relationships between molecular size, age, and “labile” DOC distributions in the ocean.