A study of the remineralization of organic carbon in nearshore sediments using carbon isotopes

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1986A study of the remineralization of organic carbon was conducted in the organic-rich sediments of Buzzards Bay, MA. Major processes affecting the carbon chemistry in sediments are reflected by changes in the stable carbon isotope ratios of dissolved inorganic carbon (ΣCO2) in sediment pore water. Six cores were collected seasonally over a period of two years. The following species were measured in the pore waters: ΣCO2, δ13C-ΣCO2, PO4, ΣH2S, Alk, DOC, and Ca. Measurements of pore water collected seasonally show large gradients with depth, which are larger in summer than in winter. The δ13C (PDB) of ΣCO2 varies from 1.3 o/oo in the bottom water to approximately -10 o/oo at 30 cm. During all seasons, there was a trend towards more negative values with depth in the upper 8 cm due to the remineralization of organic matter. There was a trend toward more positive values below 8 cm, most likely due to biological irrigation of sediments with bottom water. Below 16-20 cm, a negative gradient was re-established which indicates a return to remineralization as the main process affecting pore water chemistry. Using the ΣCO2 depth profile, it was estimated that 67-85 gC/m2 are oxidized annually and 5 gC/m2-yr are buried. The amount of carbon oxidized represented remineralization occurring within the sediments. This estimate indicated that approximately 20% of the annual primary productivity reached the sediments. The calculated remineralization rates varied seasonally with the high of 7.5 x 10-9 mol/L-sec observed in August 84 and the low (0.6 x 10-9) in December 83. The calculated remineralization rates were dependent on the amount of irrigation in the sediments; if the irrigation parameter is known to ±20%, then the remineralization rates are known to this certainty also. The amount of irrigation in the sediments was estimated using the results of a seasonal study of 222Rn/22R6a disequilibria at the same study site (Martin, 1985). Estimates of the annual remineralization in the sediments using solid-phase data indicated that the solid-phase profiles were not at steady-state concentrations. The isotopic signature of ΣCO2 was used as an indicator of the processes affecting ΣCO2 in pore water. During every month, the oxidation of organic carbon to CO2 provided over half of the carbon added to the ΣCO2 pool. However, in every month, the δ13C of ΣCO2 added to the pore water in the surface sediments was greater than -15 o/oo, significantly greater than the δ13C of solid-phase organic carbon in the sediments (-20.6 o/oo). The δ13C of ΣCO2 added to the pore water in the sediments deeper than 7 cm was between -20 and -21 o/oo, similar to the organic carbon in the sediments. Possible explanations of the 13C-enrichment observed in the surface sediments were: a) significant dissolution of CaC0, (δ13C = + 1.7 o/oo), b) the addition of significant amounts of carbonate ion from bottom water to pore water, c) an isotopic difference between the carbon oxidized in the sediments and that remaining in the sediments. The effect of CaCO3 dissolution was quantified using measured dissolved Ca profiles and was not large enough to explain the observed isotopic enrichment. An additional source of 13C-enriched carbon was bottom water carbonate ion. In every month studied, there was a net flux of ΣCO2 from pore water to bottom water. The flux of pore water ΣCO2 to bottom water ranged from a minimum of 10 x 10-12 mol/cm2-sec in December 83 to a maximum of 50 x 10-12 mol/cm2-sec in August 84. However, because the pH of bottom water was about 8 while that of the pore water was less than or equal to 7, the relative proportion of the different species of inorganic carbon (H2CO*3, HCO-3, CO2-3 was very different in bottom water and pore water. Thus, while there was a net flux of ΣCO2 from pore water to bottom water, there was a flux of carbonate ion from bottom water to pore water. Because bottom water ΣCO2 was more 13C-enriched than pore water ΣCO2, the transfer of bottom water carbonate ion to pore water was a source of 13C-enriched carbon to the pore water. If the δ13C of CO2 added to the pore water from the oxidation of organic carbon was -20.6 o/oo, then the flux of Co2-3 from bottom water to pore water must have been 10-30% of the total flux of ΣCO2 from pore water to bottom water. This is consistent with the amount calculated from the observed gradient in carbonate ion. Laboratory experiments were conducted to determine whether the δ13C of CO2 produced from the oxidation of organic carbon (δ13C-OCox) was different from the δ13C of organic carbon in the sediments (δ13C-SOC). In the laboratory experiments, mud from the sampling site was incubated at a constant temperature. Three depths were studied (0-3, 10-15, and 20-25 cm). For the first study (IE1), sediment was stirred to homogenize it before packing into centrifuge tubes for incubation. For the second study (IE2), sediment was introduced directly into glass incubation tubes by subcoring. The second procedure greatly reduced disturbance to the sediment. Rates of CO2 production were calculated from the concentrations of ΣCO2 measured over up to 46 days. In both studies, the values of Rc in the deeper intervals were about 10% of the surface values. This was consistent with the field results, although the rates decreased more rapidly in the field. In all cases, the remineralization rates during the beginning of IE1 were much greater than those at the beginning of IE2. The sediment for IE1 was collected in February 84. The measured value of Rc in the surface sediment of the laboratory experiment (24 x 10-9 mol/L-sec) was much greater than the value of Rc observed in the field in another winter month, December 83 (.62 x 10-9). The sediment for IE2 was collected in August 85. The measured values of Rc in the surface sediment (6.6-12 x 10-9 mol/L-sec) were consistent with the field values from August 84 (7.5 x 10-9). The ΣCO2 results indicated that IE2 reproduced field conditions more accurately than IE1 did. The isotopic results from the experiments strongly suggested that δ13C-OCox in the surface sediments (-17.8 o/oo ± 1.9 o/oo) was greater than δ13C-SOC (-20.6 ± 0.2 o/oo). The magnitude of the observed fractionation was small enough that the observed values of δ13C-ΣCO2 in the pore waters could be explained by fractionated oxidation coupled with the diffusion of carbonate ion from bottom water to pore water. The observed fractionation was most likely due to the multiple sources of organic carbon to coastal sediments. A study of the natural levels of radiocarbon In these sediments indicated that the carbon preserved in the sediments is approximately 30% terrestrial while the rest is from phytoplankton.Financial support was provided by the Education Office of the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program In Oceanography, by an Andrew W. Mellon Foundation grant to the Coastal Research Center, WHOI, and by the National Science foundation under grant NSF OCE83-15412

Radiocarbon measurements in the Indian Ocean aboard RVIB Nathaniel B. Palmer

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 3 (2012): 152-153, doi:10.5670/oceanog.2012.89.Research Vessel Icebreaker Nathaniel B. Palmer departed Cape Town, South Africa, on May 3, 1996, to complete the Indian Ocean portion of the "S04" line, a circumnavigation of Antarctica that was part of the US contribution to the World Ocean Circulation Experiment (WOCE). The WOCE Line S04I voyage ended at Hobart, Tasmania, on July 4, 1996, following completion of 108 stations, despite suspension of science operations for seven days on June 8, when the Palmer was diverted to deliver emergency food supplies to Russia's Mirny Station in the Davis Sea. During this extreme south cruise, with Thomas Whitworth III (Texas A&M University) and James H. Swift (Scripps Institution of Oceanography) as co-chief scientists, a total of 816 radiocarbon samples were collected by author Key at 31 stations, and these samples were later analyzed by author McNichol at the National Ocean Sciences Accelerator Mass Spectrometry Facility at the Woods Hole Oceanographic Institution

Transfer of organic carbon through marine water columns to sediments – insights from stable and radiocarbon isotopes of lipid biomarkers

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 11 (2014): 6895-6914, doi:10.5194/bg-11-6895-2014.Compound-specific 13C and 14C compositions of diverse lipid biomarkers (fatty acids, alkenones, hydrocarbons, sterols and fatty alcohols) were measured in sinking particulate matter collected in sediment traps and from underlying surface sediments in the Black Sea, the Arabian Sea and the Ross Sea. The goal was to develop a multiparameter approach to constrain relative inputs of organic carbon (OC) from marine biomass, terrigenous vascular-plant and relict-kerogen sources. Using an isotope mass balance, we calculate that marine biomass in sediment trap material from the Black Sea and Arabian Sea accounted for 66–100% of OC, with lower terrigenous (3–8%) and relict (4–16%) contributions. Marine biomass in sediments constituted lower proportions of OC (66–90%), with consequentially higher proportions of terrigenous and relict carbon (3–17 and 7–13%, respectively). Ross Sea data were insufficient to allow similar mass balance calculations. These results suggest that, whereas particulate organic carbon is overwhelmingly marine in origin, pre-aged allochthonous terrigenous and relict OC become proportionally more important in sediments, consistent with pre-aged OC being better preserved during vertical transport to and burial at the seafloor than the upper-ocean-derived marine OC.Grants OCE-9310364 and OCE-9911678 from the US National Science Foundation (NSF) and the NSF Cooperative Agreement for the Operation of a National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE-0753487 and OCE-123966) supported this research. S. G. Wakeham acknowledges the Hanse Wissenschaftskolleg (Hanse Institute for Advanced Studies), Delmenhorst, Germany, for a fellowship that supported the writing of this manuscript

Global ocean radiocarbon programs

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in McNichol, A., Key, R., & Guilderson, T. Global ocean radiocarbon programs. Radiocarbon, (2022): 1–13, https://doi.org/10.1017/rdc.2022.17.The importance of studying the radiocarbon content of dissolved inorganic carbon (DI14C) in the oceans has been recognized for decades. Starting with the GEOSECS program in the 1970s, 14C sampling has been a part of most global survey programs. Early results were used to study air-sea gas exchange while the more recent results are critical for helping calibrate ocean general circulation models used to study the effects of climate change. Here we summarize the major programs and discuss some of the important insights the results are starting to provide.Authors received funding from the National Science Foundation OCE-85865400 (APM) and a Woods Hole Oceanographic Technical Staff Award (APM)

Changes in oceanic radiocarbon and CFCs since the 1990s

Anthropogenic perturbations from fossil fuel burning, nuclear bomb testing, and chlorofluorocarbon (CFC) use have created useful transient tracers of ocean circulation. The atmospheric 14C/C ratio (∆14C) peaked in the early 1960s and has decreased now to pre-industrial levels, while atmospheric CFC-11 and CFC-12 concentrations peaked in the early 1990s and early 2000s, respectively, and have now decreased by 10%–20%. We present the first analysis of a decade of new observations (2007 to 2018–2019) and give a comprehensive overview of the changes in ocean ∆14C and CFC concentration since the WOCE surveys in the 1990s. Surface ocean ∆14C decreased at a nearly constant rate from the 1990–2010s (20‰/decade). In most of the surface ocean ∆14C is higher than in atmospheric CO2 while in the interior ocean, only a few places are found to have increases in ∆14C, indicating that globally, oceanic bomb 14C uptake has stopped and reversed. Decreases in surface ocean CFC-11 started between the 1990 and 2000s, and CFC-12 between the 2000–2010s. Strong coherence in model biases of decadal changes in all tracers in the Southern Ocean suggest ventilation of Antarctic Intermediate Water was enhanced from the 1990 to the 2000s, whereas ventilation of Subantarctic Mode Water was enhanced from the 2000 to the 2010s. The decrease in surface tracers globally between the 2000 and 2010s is consistently stronger in observations than in models, indicating a reduction in vertical transport and mixing due to stratification

Radiocarbon content of dissolved organic carbon in the South Indian Ocean

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 872–879, doi:10.1002/2017GL076295.We report four profiles of the radiocarbon content of dissolved organic carbon (DOC) spanning the South Indian Ocean (SIO), ranging from the Polar Front (56°S) to the subtropics (29°S). Surface waters held mean DOC Δ14C values of −426 ± 6‰ (~4,400 14C years) at the Polar Front and DOC Δ14C values of −252 ± 22‰ (~2,000 14C years) in the subtropics. At depth, Circumpolar Deep Waters held DOC Δ14C values of −491 ± 13‰ (~5,400 years), while values in Indian Deep Water were more depleted, holding DOC Δ14C values of −503 ± 8‰ (~5,600 14C years). High-salinity North Atlantic Deep Water intruding into the deep SIO had a distinctly less depleted DOC Δ14C value of −481 ± 8‰ (~5,100 14C years). We use multiple linear regression to assess the dynamics of DOC Δ14C values in the deep Indian Ocean, finding that their distribution is characteristic of water masses in that region.National Science Foundation (NSF) Grant Numbers: OPP-1142117, OCE-14367482018-07-2

Constraining the sources and cycling of dissolved organic carbon in a large oligotrophic lake using radiocarbon analyses

© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 208 (2017): 102-118, doi:10.1016/j.gca.2017.03.021.We measured the concentrations and isotopic compositions of solid phase extracted (SPE) dissolved organic carbon (DOC) and high molecular weight (HMW) DOC and their constituent organic components in order to better constrain the sources and cycling of DOC in a large oligotrophic lacustrine system (Lake Superior, North America). SPE DOC constituted a significant proportion (41-71 %) of the lake DOC relative to HMW DOC (10-13%). Substantial contribution of 14C-depleted components to both SPE DOC (Δ14C = 25 to 43‰) and HMW DOC (Δ14C = 22 to 32‰) was evident during spring mixing, and depressed their radiocarbon values relative to the lake dissolved inorganic carbon (DIC; Δ14C ~ 59‰). There was preferential removal of 14C-depleted (older) and thermally recalcitrant components from HMW DOC and SPE DOC in the summer. Contemporary photoautotrophic addition to HMW DOC was observed during summer stratification in contrast to SPE DOC, which decreased in concentration during stratification. Serial thermal oxidation radiocarbon analysis revealed a diversity of sources (both contemporary and older) within the SPE DOC, and also showed distinct components within the HMW DOC. The thermally labile components of HMW DOC were 14C-enriched and are attributed to heteropolysaccharides (HPS), peptides/amide and amino sugars (AMS) relative to the thermally recalcitrant components reflecting the presence of older material, perhaps carboxylic-rich alicyclic molecules (CRAM). The solvent extractable lipid-like fraction of HMW DOC was very 14C-depleted (as old as 1270-2320 14C years) relative to the carbohydrate-like and protein-like substances isolated by acid hydrolysis of HMW DOC. Our data constrain relative influences of contemporary DOC and old DOC, and DOC cycling in a modern freshwater ecosystem.This work was funded by the National Science Foundation OCE 0825600 to E.C.M. and J.P.W., a graduate student internship fellowship to P.K.Z by National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE 0753487), and the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution to P.K.Z, with funding provided by the National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE 0753487)

The passage of the bomb radiocarbon pulse into the Pacific Ocean

Author Posting. © Arizona Board of Regents on behalf of the University of Arizona, 2010. This article is posted here by permission of Dept. of Geosciences, University of Arizona for personal use, not for redistribution. The definitive version was published in Radiocarbon 52 (2010): 1182-1190.We report and compare radiocarbon observations made on 2 meridional oceanographic sections along 150°W in the South Pacific in 1991 and 2005. The distributions reflect the progressive penetration of nuclear weapons-produced 14C into the oceanic thermocline. The changes over the 14 yr between occupations are demonstrably large relative to any possible drift in our analytical standardization. The computed difference field based on the gridded data in the upper 1600 m of the section exhibits a significant decrease over time (approaching 40 to 50‰ in Δ14C) in the upper 200–300 m, consistent with the decadal post-bomb decline in atmospheric 14C levels. A strong positive anomaly (increase with time), centered on the low salinity core of the Antarctic Intermediate Water (AAIW), approaches 50–60‰ in Δ14C, a clear signature of the downstream evolution of the 14C transient in this water mass. We use this observation to estimate the transit time of AAIW from its “source region” in the southeast South Pacific and to compute the effective reservoir age of this water mass. The 2 sections show small but significant changes in the abyssal 14C distributions. Between 1991 and 2005, Δ14C has increased by 9‰ below 2000 m north of 55°S. This change is accompanied overall by a modest increase in salinity and dissolved oxygen, as well as a slight decrease in dissolved silica. Such changes are indicative of greater ventilation. Calculation of “phosphate star” also indicates that this may be due to a shift from the Southern Ocean toward North Atlantic Deep Water as the ventilation source of the abyssal South Pacific.This work was performed under National Science Foundation Grant number OCE-0223434 as well as a cooperative agreement with NSF (most recently OCE-0228996)

Carbon isotopic evidence for microbial control of carbon supply to Orca Basin at the seawater–brine interface

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 10 (2013): 3175-3183, doi:10.5194/bg-10-3175-2013.Orca Basin, an intraslope basin on the Texas-Louisiana continental slope, hosts a hypersaline, anoxic brine in its lowermost 200 m in which limited microbial activity has been reported. This brine contains a large reservoir of reduced and aged carbon, and appears to be stable at decadal time scales: concentrations and isotopic composition of dissolved inorganic (DIC) and organic carbon (DOC) are similar to measurements made in the 1970s. Both DIC and DOC are more "aged" within the brine pool than in overlying water, and the isotopic contrast between brine carbon and seawater carbon is much greater for DIC than DOC. While the stable carbon isotopic composition of brine DIC points towards a combination of methane and organic carbon remineralization as its source, radiocarbon and box model results point to the brine interface as the major source region for DIC, allowing for only limited oxidation of methane diffusing upwards from sediments. This conclusion is consistent with previous studies that identify the seawater–brine interface as the focus of microbial activity associated with Orca Basin brine. Isotopic similarities between DIC and DOC suggest a different relationship between these two carbon reservoirs than is typically observed in deep ocean basins. Radiocarbon values implicate the seawater–brine interface region as the likely source region for DOC to the brine as well as DIC.This work was funded by the WHOI Postdoctoral Scholar program, NSF Cooperative Agreement for the Operation of a National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE-0753487), and the US National Science Foundation’s Emerging Frontiers program (award 0801741 to SBJ)

Rapid extraction of dissolved inorganic carbon from seawater and groundwater samples for radiocarbon dating

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography: Methods 14 (2016): 24-30, doi:10.1002/lom3.10066.We designed and developed a system to efficiently extract dissolved inorganic carbon (DIC) from seawater and groundwater samples for radiocarbon dating. The Rapid Extraction of Dissolved Inorganic Carbon System (REDICS) utilizes a gas-permeable polymer membrane contactor to extract the DIC from an acidified water sample in the form of carbon dioxide (CO2), introduce it to a helium gas stream, cryogenically isolate it, and store it for stable and radiocarbon isotope analysis. The REDICS system offers multiple advantages to the DIC extraction method which has been used for the last several decades at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) at the Woods Hole Oceanographic Institution, including faster DIC extraction, streamlined analysis, and minimized set-up and prep time. The system was tested using sodium carbonate and seawater standards, duplicates of which were also processed on the water stripping line (WSL) at NOSAMS. The results demonstrate that the system successfully extracts, quantifies, and stores more than 99% of the DIC in less than 20 min. Stable and radiocarbon isotope analysis demonstrated system precision of 0.04‰ and 7.8‰, respectively. A Sargasso Sea depth profile was used to further validate the system. The results show high precision for both stable and radiocarbon analysis with pooled standard deviations of 0.02‰ and 5.6‰, respectively. A comparison between the REDICS and WSL analyses indicates a good accuracy for both stable and radio-isotope analysis.NSF Cooperative Agreements for the Operation of a National Ocean Sciences Accelerator Mass Spectrometry Facility (OCE-0753487 and OCE-123966) supported this research