G03_01

Rock-leached organic carbon drives subsurface microbiomes

Heinze B^1,2, Schwab V², Trumbore S^2,3, Xu X³, Schroeter S², Chaudhari N^1,4, Küsel K^1,4

¹Aquatic Geomicrobiology, Institute of Biodiversity, Friedrich Schiller University, Jena, Germany, ²Department Biogeochemical Processes, Max-Planck-Institute for Biogeochemistry, Jena, Germany, ³Department of Earth System Science, University of California, Irvine, USA, ⁴German Center for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Germany

More than 99% of global carbon is stored in sedimentary rocks, with an estimated 0.1 Gt CO₂ released every year due to weathering of rock organic carbon. However, little is known about the role of sedimentary rocks as source of energy and carbon in subsurface microbiomes. To study the extent to which rock-derived organic carbon is driving microbial activities in aquifers, we incubated crushed carbonate rocks rich in alkanes or aromatics with groundwaters under oxic and anoxic conditions. Rocks immediately leached aromatic and aliphatic substances into the groundwater, leading to a 50% increase in dissolved organic carbon (DOC). Interestingly, the Δ¹⁴C-signature of the DOC was in the range of -322 to -447‰ – ‘young’ compared to the ¹⁴C-dead rock-kerogen, suggesting the addition of younger, fresher organic materials along surfaces on rock fractures exposed to groundwater. The ¹⁴C-content of membrane lipids extracted from the POC of the incubations indicated that most of their C (96-100% in anoxic and 44-77% in oxic groundwaters) were derived from the younger rock leachate (i.e. DOC) rather than ¹⁴C-free sedimentary organic matter. Replicate incubations amended with ¹³C-labelled inorganic C showed little to no autotrophic C fixation over the time of incubation, excluding DIC as an important C source. Metagenomics identified potential bacterial degraders of rock-derived hydrocarbons within Desulfosporosinus and Dechloromonas (anoxic), and Rhodoferax, and Methyloversatilis (oxic). Our findings show that besides surface-derived or primarily produced C, organic C leached from sedimentary rocks can drive subsurface ecosystem metabolism, underlining the importance of microorganisms in geogenic C turnover.

G03_02

Time-series measurements of dissolved organic and inorganic radiocarbon from Switzerland’s two largest lakes

White M¹, Mittelbach B¹, Rhyner T¹, Haghipour N¹, Blattmann T¹, Jacquin C², Schubert C³, Wessels M⁴, Dubois N^1,3, Eglinton T¹

¹Department of Earth Sciences, ETH Zürich, Zürich, Switzerland, ²Department of Process Engineering, Eawag, Dübendorf, Switzerland, ³Department Surface Waters Research & Management, Eawag, , Switzerland, ⁴Institut für Seenforschung der LUBW, Langenargen, Germany

The Radiocarbon Inventories of Switzerland (RICH) project aims to construct the first national-scale census of carbon across aquatic, terrestrial, and atmospheric reservoirs. Within the Swiss carbon cycle, inland waters play a crucial role with lakes integrating carbon from various sources within their catchment in addition to that fixed by local primary productivity. Here we present measurements of dissolved organic and inorganic radiocarbon from monthly water column samplings of Switzerland’s two largest lakes: Lake Constance and Lake Geneva. Such high-resolution temporal measurements can uncover the nature and drivers of seasonal carbon dynamics in these well-studied lake ecosystems. Comparison of water column profiles from river proximal and river distal sites within each lake will constrain fluvial influence. In addition to isotope data, measurements of the optical properties of dissolved organic matter will aid in untangling sources and cycling of lake water DOC. Preliminary results show that the average radiocarbon signature of DIC in both lakes is depleted relative to atmospheric CO₂, suggesting a ca. 15% contribution from weathering of petrogenic (rock-derived) carbon.

G03_03

Sedimentary radiocarbon signatures reveal persistent input of soil organic matter into Lake Constance

Mittelbach B¹, White  M¹, Rhyner T¹, Blattmann T¹, Haghipour N¹, Wessels M³, Dubois N², Eglinton T¹

¹ETH Zurich, Zürich, Switzerland, ²Eawag, Dübendorf, Switzerland , ³Institut für Seenforschung der LUBW, Langenargen, Germany

Inland waters play a crucial role in the global carbon cycle, as the sequestration of organic carbon (OC) in lake sediments constitutes a sink of atmospheric CO₂. Notably, the source of this sequestered OC has important implications for carbon cycling and climate. The burial of recently synthesized terrestrial and aquatic biospheric OC and of aged, soil-derived OC represents a drawdown of atmospheric CO₂. In contrast, erosion, transport, and reburial of rock-derived OC exert no net effect on atmospheric CO₂ levels.

Radiocarbon can be used to differentiate between these different sources of OC and constrain proportions of recent, pre-aged,  and fossil carbon. Moreover, the  20th-Century  radiocarbon “bomb spike” offers the possibility to resolve organic matter turnover and transport on decadal timescales. We combine ¹⁴C and stable δ¹³C isotope signatures of bulk OC from sediment cores retrieved from perialpine Lake Constance to assess the nature and dynamics of OC accumulation over the last century. Information about OC source isotope signatures was gathered from soil and bedrock profiles in the catchment as well as sediment traps in the lake. 

Our results show a muted, but distinct, bomb spike with Δ¹⁴C values of bulk OC increasing from -250‰ to -100‰ in the early 1960s. A linear mixing model reveals that soil-derived, pre-aged carbon comprises the largest contributor to the bulk sedimentary OC pool. We attribute the presence of the bomb spike signal to inputs of aquatic biomass, arguing for a rapid incorporation of bomb carbon into the lake DIC pool.

G03_P01

Source apportionment of fugitive methane emissions using radiocarbon in a Scottish river.

Gulliver P¹, Ascough P¹, Murray  C¹, Taylor C¹, Waldron S²

¹University Of Glasgow, East Kilbride, United Kingdom, ²University of Glasgow , Glasgow, United Kingdom

There is a long history of underground and open cast mining in Scotland. Closure of these mines results in local water table rebound as the mines slowly fill with water. This affects a number of Scottish river catchments and the escape of rebounding mine water provides a simple explanation for the presence of high methane concentrations in spring waters in such a catchment.

However, radiocarbon analysis of the dissolved methane in multiple springs of a Scottish catchment underlain by abandoned mine workings gave results of approximate 70 % modern, clearly showing that the methane is not solely geologically derived.

Radiocarbon analysis of the dissolved inorganic and organic carbon (DIC and DOC respectively) pools at the same sites showed that despite having lower % modern values then the dissolved methane, neither pool had a solely geological signature.

The source organic matter contributing to the majority of the dissolved methane and a significant proportion of the DIC and DOC comes from a much younger source and is biologically produced.

G03_P02

Bulk Organic Carbon Isotopes From the Santa Clara River During Rain Events

Thomas K¹, Hauksson N¹, Druffel E¹

¹UC Irvine, Irvine, United States

Small mountainous rivers leading into the ocean, such as the Santa Clara River in Southern California, export considerable amounts of organic carbon to the ocean, and the magnitude and composition of which reaches the ocean is widely studied. With rainfall events in California mostly during winter, the composition and ¹⁴C age of the organic carbon deposited by rivers is likely dependent on the magnitude of these events. In this study, we measured Δ¹⁴C and δ¹³C values of sedimentary organic carbon (SOC) and particulate organic carbon (POC) after rain events near the Santa Clara River. We present these analyses that show high variability of ¹⁴C age and composition of organic material moved downstream with the amount of precipitation and runoff entering the river. This study contributes to our knowledge of organic carbon cycling from small mountainous rivers.

G03_P03

Anthropogenic perturbations change the quality and quantity of terrestrial carbon flux to the coastal ocean

Wei B^1,2, Mollenhauer G^1,3,4, Kusch S⁴, Hefter J¹, Grotheer H¹, Schefuß E⁴, Geibert W¹, Ransby D¹, Jia G^1,6

¹Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany, ²State Key Laboratory of Marine Geology, Tongji University, Shanghai, China, ³Department of Geosciences, University of Bremen, Bremen, Germany, ⁴MARUM-Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany, ⁵University of Cologne-Centre for Accelerator Mass Spectrometry, University of Cologne, Cologne, Germany, ⁶Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China

Rivers export organic carbon (OC) sourced from their watersheds, offering an opportunity to assess impacts of climatic change and anthropogenic perturbations on the transfer of terrestrial OC to the ocean. Using ¹³C and ¹⁴C compositions of OC exported by the Pearl River during the Industrial Age, we show that anthropogenic activities primarily control the quantity and quality of terrestrial OC fluxes to the coastal ocean. Damming the river and accelerating coal consumption have led to increasing burial flux of petrogenic OC, a rather stable carbon fraction. Man-made ecosystem changes including deforestation, cropland extension, urbanization, and river management increased fresh terrestrial biospheric OC burial, additionally contributing to the long-term carbon sink. Our data help identifying the drivers of sustained change in terrestrial OC export and reveal that human activities substantially enhance the transfer of petrogenic OC and fresh biospheric OC to the coastal ocean, acting as an important sink for anthropogenic CO₂.