Wildfires and fossil fuel combustion produce large amounts of charred pyrogenic organic materials, including black carbon (BC). Pyrogenic carbon is chemically heterogenous and is described as a continuum from charcoal to soot. Black carbon is an incomplete combustion byproduct that could be a sink for fixed carbon, especially when deposited to pelagic sediments or long-term cycling in the oceans. In the atmosphere, BC is a key driver of global climate change; yet it’s feedbacks are not included in Earth System Models. Omitting BC production from assessments of fire impacts leads to an overestimation of the strength of positive feedbacks caused by wildfires. There is a general assumption that rivers deliver most or all BC to the ocean. Yet the sources, transport and transformation of BC are not well constrained in intermediate reservoirs or in atmospheric and ocean circulation models. For example, few BC flux measurements are available in remote ocean sediments to understand this transfer of BC. Once deposited to the ocean, some BC is buried in sediments, and can account for significant fractions of the organic matter preserved in sediments. Dissolved BC, in contrast, cycles in the oceans on millennial timescales. A better understanding of the carbon cycle thus depends on working at the interface of numerous disciplines, including oceanography, paleography, atmospheric science, biogeochemistry, and analytical chemistry. The proposed ASLO session is thus relevant and timely to further expand our understanding of the cycling of pyrogenic carbon, as well as BC components, which is key to understanding carbon mitigation and negative feedback strategies with continued climate change.
Lead Organizer: Rainer Lohmann, University of Rhode Island (rlohmann@uri.edu)
Co-organizers:
Sasha Wagner, Rensselaer Polytechnic Institute (wagnes3@rpi.edu)
Alysha Coppola, ETH Zurich (acoppola@ethz.ch)
Presentations
10:30 AM
DEEP HYDROTHERMAL SYSTEMS AS A SOURCE OF DISSOLVED BLACK CARBON IN THE OCEAN (4674)
Primary Presenter: Youhei Yamashita, Hokkaido Univeristy (yamashiy@ees.hokudai.ac.jp)
Pyrogenic carbon, a byproduct of biomass and fossil fuel combustion, is an important component in global carbon cycle because it can be stored on Earth’s surface for centuries to millennia. The dissolvable condensed aromatic fraction of pyrogenic carbon occurs ubiquitously in aquatic environments on Earth’s surface and is called dissolved black carbon (DBC). DBC is known to occur ubiquitously in the ocean, but the DBC cycle and budget in the ocean have not been well constrained due to the paucity of observations on basin-scale distributions. Deep hydrothermal vents are suggested to be apossible source of thermogenic DBC, but little is known about the distribution of hydrothermal DBC in the deep ocean. Here, we show the basin-scale distributions of DBC along two transects in the eastern Pacific Ocean. The concentration and condensation degree of deep ocean DBC at the sites close to the East Pacific Rise were higher than those at the other sites, suggesting that highly condensed DBC was derived from hydrothermal systems and transported over long distances. The hydrothermal DBC concentration was quantified as a deviation from the linear relationship between the DBC concentration and apparent oxygen utilization previously observed in the central and western Pacific Ocean. The deviation was linearly correlated with excess 3He, a tracer of hydrothermal input, confirming the hydrothermal origin of the DBC.
10:45 AM
Are hydrothermal subsurface sediments a source of recalcitrant dissolved organic matter to the deep ocean? (5765)
Primary Presenter: Melina Knoke, University of Oldenburg (melina.knoke@uni-oldenburg.de)
Dissolved organic sulfur (DOS) and dissolved black carbon (DBC) are quantitatively important fractions of the global recalcitrant marine dissolved organic matter (DOM) pool. DOS is formed by marine microorganisms and abiotic sulfurization. DBC is mainly produced during incomplete combustion of biomass on land and marine asphalt seepage was identified to contribute to the oceanic DBC and DOS pools. However, the hydrothermal sources are still not well constrained. We hypothesize that DOS and DBC are mobilized from deep subsurface sediments via hydrothermal heating. The Guaymas Basin (GB) is ideal to study these processes since sill intrusions into the sediments result in thermal alteration of organic matter and thus pore water chemistry. We investigated pore water and sediments collected during IODP Expedition 385. DOM was characterized molecularly using ultrahigh-resolution mass spectrometry (FT-ICR-MS). DOS concentrations were quantified with elemental analysis and DBC as benzenepolycarboxylic acids. Our results revealed three reaction zones in the GB subsurface: the upper sediments were characterized by active microbial decomposition of DOM. In the deeper sediments, DOM accumulated, partly due to in situ formation of DOS and BC solubilization from sediments. At the sill contact zone, heat-induced molecular alterations reduced the DOM molecular diversity. We conclude that hydrothermal heating stimulates the release and formation of recalcitrant DOM in the GB sediments which can be released to the deep ocean during advective hydrothermal events.
11:00 AM
Effects of decadal-scale environmental aging on wildfire char-derived dissolved organic matter yields and composition (6233)
Primary Presenter: Andrew Wozniak, University of Delaware (awozniak@udel.edu)
Recent work demonstrates that a leachable fraction of pyrogenic organic matter (PyOM, aka ‘black carbon’) is transported hydrologically to the ocean where it contributes to the long-cycling pool of dissolved organic carbon. Like PyOM, dissolved PyOM (PyDOM) properties vary predictably along a combustion continuum, but the effects of environmental aging on PyDOM characteristics are not well-established. Wildfire-derived chars aged for 1, 4, and 30 (GDS-30) years were collected from the Great Dismal Swamp, USA and analyzed for their solid and aqueous leachate chemical characteristics. Solid 13C NMR spectroscopic and elemental analyses show GDS-30 to have the highest %O and lowest %C and aromatic C contents suggesting it to have the lowest charring extent of the 3 study chars. However, benzenepolycarboxylic acid (BPCA) marker analyses reveal GDS-30 to have the highest BPCA yield and highest degree of aromatic condensation consistent with the opposite conclusion (highest charring extent). In PyDOM leachates, increased char aging correlates with increased DOC and BPCA yields and FTICR-MS-derived aromatic and low O/C molecular formulas. We hypothesize that hydrological and oxidative aging processes 1) deliver allochthonous materials yielding higher PyOM H/C and O/C ratios, and 2) selectively remove an oxygenated, less aromatic portion of the initial PyOM. Subsequently, only the most highly condensed aromatic compounds are available for oxidation and transport as PyDOM, and PyDOM molecular characteristics are better explained by an aging continuum than a combustion continuum.
11:15 AM
DIFFERENT SOURCES OF CONTINENTAL ORGANIC CARBON ACCUMULATED SURFACE SEDIMENTS FROM THE TROPICAL ATLANTIC OCEAN (5895)
Primary Presenter: Nuria Penalva Arias, Autonomous University of Barcelona (UAB) (Nuria.Penalva@uab.cat)
Marine sediments represent the main reservoir for black carbon (BC) residues in the ocean, playing an essential role in the global carbon cycle. Therefore, sediments constitute an important environmental compartment for determining the distribution and emission sources of BC. The BC term encompasses a wide range of combustion continuum generated during the incomplete combustion of organic matter. The quantification of the polycondensed aromatic fraction of BC can be performed by oxidizing BC to generate benzene polycarboxylic acids (BPCAs). These compounds have been widely applied as specific markers to quantify and characterize pyrogenic carbon (PyC) in environmental matrices. In order to improve our understanding of the global carbon cycle is essential to disentangle the sources, fate and composition of the organic carbon accumulated in marine sediments. Our results show a preferential accumulation of BPCAs in sediments in a latitudinal band, which indicates an association with PyC emissions from Savannah wildfires. The isotopic signature for B6CA and B5CA indicates C4-type plants as the main source of PyC. In contrast, the isotopic signature of continentally-derived n-alkanes is consistent with C3-type plants. Therefore, the isotopic data indicates that there are two different sources of continentally-derived organic carbon. The PyC source is quantitatively dominant and is related to the burning of African savannah ecosystems. A second source of continentally derived organic matter is quantitatively less important and corresponds to C3-type plants of unknown origin.
11:30 AM
THE CONCENTRATIONS, SOURCES, AND AGE OF SEDIMENTARY BLACK CARBON IN THE EASTERN EQUATORIAL ATLANTIC OCEAN (6115)
Primary Presenter: Samuel Katz, University of Rhode Island (sdkatz@uri.edu)
Incomplete combustion of biomass is the main source of black carbon (BC) to the global oceans via aeolian and fluvial deposition. Typically, particulate BC is pre aged before reaching ocean sediments, its ultimate sink. Annual burning of crop fields in central Africa provides a large source of BC, that may be transported by winds to the equatorial Atlantic and deposited within the intertropical convergence zone. Here, we assessed whether the transport of BC from biomass burning in western Tropical Africa dominates sedimentary black carbon concentrations in the eastern equatorial Atlantic Ocean. We collected multi cores along a 5˚ N transect of the Atlantic Ocean and isolated black carbon using the chemothermal oxidation at 375 ˚C method. BC concentrations decreased westward across the Equatorial Atlantic, with the highest concentrations (0.717 mg/g) north of the Sierra Leon Rise and lowest (0.296 mg/g) west of the mid Atlantic ridge. Source identification using stable carbon isotopes of BC depicted a westward depletion from C4 plants towards C3 plants or fossil fuel dominated values (-17.36 ‰ to -21.02 ‰). The age of surface sedimentary BC in the eastern equatorial Atlantic was younger than the Total Organic Carbon (TOC) (BC: 1460 ± 15 yrs. vs. TOC: 2180 ± 20 yrs.). At least in the eastern equatorial Atlantic, BC had a unique signature from C4 plants that is relatively young and likely originating from biomass burning in Africa. This unique isotopic pattern could be used to further the understanding the transport and fate of black carbon’s role in the carbon cycle.
11:45 AM
ISOTOPIC EVIDENCE OF AGING OF PARTICULATE BLACK CARBON IN THE OPEN OCEAN (4809)
Primary Presenter: Siddhartha Sarkar, Physical Research Laboratory (siddhartha.academia@gmail.com)
Black carbon (BC) has emerged as an integral part of the global carbon (C) cycle with a potential to generate negative climate feedbacks and sequester C at long timescales. There is a limited understanding of the pathways of transformation of BC in aquatic systems. Photodegradation of BC along the land-ocean continuum provides a fresh perspective of their role in aquatic C biogeochemistry and to the uncertainty in the global C budget. However, to reach to certain conclusions, extensive research is warranted in the field of BC cycling. Although research on dissolved black carbon has accelerated in recent years, their particulate counterpart remains largely unexplored. In this study, we investigated the particulate black carbon (PBC) dynamics in the estuarine and marine settings of the Northern Indian Ocean by measuring its concentrations and isotopic compositions (δ13CPBC). after removing inorganic and organic C fractions from the collected particulate samples (on 0.7 µm Whatman GF/F) using HCl fumigation and CTO-375 method, respectively. Significantly higher δ13CPBC in the open ocean (Arabian Sea: –16.2 ± 1.3‰ and Bay of Bengal: –20.0 ± 0.7‰) compared to estuarine (Andaman: –24.5 ± 1.1‰; Mahanadi: –25.0 ± 1.3‰), riverine (Ambika: –26.9‰) and atmospheric (–25.5 and –27.8‰) environments was observed. This unequivocal increase in δ13CPBC indicated potential aging of PBC in the seawater through processes like photodegradation or leaching. This finding adds new dimension to the PBC cycling in our environment which might have implications on BC burial at longer timescale.
SS041 Transport, Transformation and Trends of Pyrogenic Carbon in the Aquatic Environment
Description
Time: 10:30 AM
Date: 7/6/2023
Room: Sala Portixol 1