Fire and Water

Reconciling the black carbon conundrum at the river-to-ocean interface

Forest fires release large amounts of carbon into the atmosphere that are changing Earth’s climate. Up to a third of carbon from fires is thought to be locked away as a residue called Black carbon (BC) rather than emitted as greenhouse gases. BC plays important role in the global carbon cycle because it a very large and presumably stable component. However, budgets are unconstrained as there is an imbalance between what is produced on land, and where it ends up in the oceans. Here, I address the role of rivers, as connecting terrestrial and marine carbon cycles. I focus on the Mackenzie River in the Arctic because it is rapidly changing with climate change, as large stores of carbon from land are being released into the Beaufort Sea. Through a holistic “source-to-sink” approach using in-situ experiments, assessments and modeling studies, I address the information void at this river-ocean continuum to address if BC is really locked away, or if BC is degraded at these dynamic interfaces.

The approach

Biomass burning and fossil fuel combustion releases large amounts of carbon into the atmosphere that are changing Earth’s climate. Up to 27% and 0.2% of carbon from biomass burning and fossil fuel combustion, respectively, is retained as a byproduct from incomplete combustion (Black carbon, BC) rather than emitted as greenhouse gases. In addition to affecting radiative budgets, BC also influences biogeochemical processes, and plays an important role in the global carbon cycle because it a very large component. For example, BC acts as a biospheric carbon sink (by removing carbon from faster atmosphere-biosphere processes and sequestering this carbon in soil and sedimentary reservoirs). To predict how the carbon cycle will respond to climate change, we need to determine the origin, dynamics and fate of this abundant and slowly-cycling component in the carbon cycle. Our understanding of the role of BC in the regional and global-scale carbon cycle is far from complete, due in large part to poor constraints on the river export and fate of BC in the ocean. Current estimates suggest that the input by rivers alone to the ocean is sufficient to sustain the turnover of the entire oceanic BC pool in just 500 14C years given current known losses of BC, yet measured 14C ages of BC in the deep sea are 40 times greater (>20,000 14C years). This imbalance suggests we are underestimating the losses of BC, but where, how and why these losses occur remains unknown. Resolving this conundrum is essential for determining the significance and responsiveness of BC as a sink of atmospheric CO2. In order to fill this information gap, we need improved constraints on the origin, dynamics and fate of this elusive but critical component of the C cycle.

Here, we propose an approach that seeks to bridge this crucial knowledge gap by constraining the magnitude and nature of BC transfer across the river-to-ocean aquatic interface, using the Mackenzie River-Beaufort Sea as a model system. To do so, we will evaluate the abundance, composition and reactivity of BC based on carbon isotopic (14C) and BC-like (polycyclic condensed aromatic) molecular signatures in both particulate and dissolved carbon pools spanning the river-ocean aquatic continuum. We will use this information to test my overarching hypothesis that BC exported from rivers is modified, degraded and re-mineralized at the coastal interface margins, with residual products becoming incorporated into and part of the recalcitrant DOC background observed in the deep ocean waters. We also hypothesize that BC is intrinsically heterogenous, but is subject to modification and degradation by a universal suite of abiotic and biotic mechanisms. Briefly, in Work Package (WP) 1, the doctoral student will focus on the river-to-margin interface. In WP-2, we will focus on the coastal ocean-open ocean interface. Finally, in WP3, I will take a holistic “source-to-sink” approach synthesizing data across the entire aquatic continuum. We will generate a simple box model for testing the hypotheses presented above. The model will incorporate information on the source and age of DBC and PBC derived from complementary 14C age, molecular composition and specific molecular markers, as well as information on BC quality or reactivity derived from field and laboratory incubation experiments (in WP 1 and 2) designed to shed light on the mechanisms and rates of BC loss and transformation by oxidation and degradation loss mechanisms. By combining geochemical characterization, in situ experiments and large-scale observations in a mass balance box model, we will bridge the information void concerning the fate of BC during transport along the aquatic river-delta-coastal ocean-open ocean continuum. The study will focus on the Arctic because of the disproportionally large terrestrial influence on this semi-enclosed basin, and because of the rapid pace of change that this system is experiencing. However, processes at the land-ocean transition that form the focus of this investigation are of global relevance with respect to the sources and fate of BC. We will shed new light on the dynamics of this important yet enigmatic component of the carbon cycle as well as its sensitivity to, and influence on past and future environmental change.

Project members

• Alysha Coppola
• Linn Speidel