Grocke Darren R.

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Darren R.

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  • Preprint
    Organic geochemistry of the early Toarcian oceanic anoxic event in Hawsker Bottoms, Yorkshire, England
    ( 2013-12) French, Katherine L. ; Sepulveda, Julio ; Trabucho-Alexandre, J. ; Grocke, Darren R. ; Summons, Roger E.
    A comprehensive organic geochemical investigation of the Hawsker Bottoms outcrop section in Yorkshire, England has provided new insights about environmental conditions leading into and during the Toarcian oceanic anoxic event (T-OAE; ~183 Ma). Rock-Eval and molecular analyses demonstrate that the section is uniformly within the early oil window. Hydrogen index (HI), organic petrography, polycyclic aromatic hydrocarbon (PAH) distributions, and tricyclic terpane ratios mark a shift to a lower relative abundance of terrigenous organic matter supplied to the sampling locality during the onset of the T-OAE and across a lithological transition. Unlike other ancient intervals of anoxia and extinction, biomarker indices of planktonic community structure do not display major changes or anomalous values. Depositional environment and redox indicators support a shift towards more reducing conditions in the sediment porewaters and the development of a seasonally stratified water column during the T-OAE. In addition to carotenoid biomarkers for green sulfur bacteria (GSB), we report the first occurrence of okenane, a marker of purple sulfur bacteria (PSB), in marine samples younger than ~1.64 Ga. Based on modern observations, a planktonic source of okenane’s precursor, okenone, would require extremely shallow photic zone euxinia (PZE) and a highly restricted depositional environment. However, due to coastal vertical mixing, the lack of planktonic okenone production in modern marine sulfidic environments, and building evidence of okenone production in mat-dwelling Chromatiaceae, we propose a sedimentary source of okenone as an alternative. Lastly, we report the first parallel compound-specific δ13C record in marine- and terrestrial-derived biomarkers across the T-OAE. The δ13C records of short-chain n-alkanes, acyclic isoprenoids, and long-chain n-alkanes all encode negative carbon isotope excursions (CIEs), and together, they support an injection of isotopically light carbon that impacted both the atmospheric and marine carbon reservoirs. To date, molecular δ13C records of the T-OAE display a negative CIE that is smaller in magnitude compared to the bulk organic δ13C excursion. Although multiple mechanisms could explain this observation, our molecular, petrographic, and Rock-Eval data suggest that variable mixing of terrigenous and marine organic matter is an important factor affecting the bulk organic δ13C records of the T-OAE.
  • Preprint
    Erosion of organic carbon in the Arctic as a geological carbon dioxide sink
    ( 2015-05-12) Hilton, Robert G. ; Galy, Valier ; Gaillardet, Jerome ; Dellinger, Mathieu ; Bryant, Charlotte ; O'Regan, Matt ; Grocke, Darren R. ; Coxall, Helen ; Bouchez, Julien ; Calmels, Damien
    Soils of the northern high latitudes store carbon over millennial timescales (103 yrs) and contain approximately double the carbon stock of the atmosphere1-3. Warming and associated permafrost thaw can expose soil organic carbon and result in mineralisation and carbon dioxide (CO2) release4-6. However, some of this soil organic carbon may be eroded and transferred to rivers7-9. If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (>104 yrs), geological CO2 sink8-10. Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify POC source in the Mackenzie River, the main sediment supplier to the Arctic Ocean11,12 and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios 26 to correct for rock-derived POC10,13,14. Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5800±800 yr, much older than large tropical rivers13,14. Based on the measured biospheric POC content and variability in annual sediment yield15, we calculate a biospheric POC flux of 𝟐. 𝟐𝟐−𝟎𝟎.𝟗𝟗 +𝟏𝟏.𝟑𝟑 TgC yr-1 from the Mackenzie River, three times the CO2 drawdown by silicate weathering16. Offshore we find evidence for efficient terrestrial organic carbon burial over the Holocene, suggesting that erosion of organic carbon-rich, high latitude soils may result in a significant geological CO2 sink.
  • Article
    Turbidity currents can dictate organic carbon fluxes across river‐fed fjords: an example from Bute Inlet (BC, Canada)
    (American Geophysical Union, 2022-05-25) Hage, Sophie ; Galy, Valier ; Cartigny, Matthieu J. B. ; Heerema, Catharina ; Heijnen, Maarten S. ; Acikalin, Sanem ; Clare‬, Michael A. ; Giesbrecht, Ian J. W. ; Grocke, Darren R. ; Hendry, A. ; Hilton, Robert G. ; Hubbard, Stephen M. ; Hunt, James E. ; Lintern, D. Gwyn ; McGhee, Claire A. ; Parsons, Daniel R. ; Pope, Edward L. ; Stacey, Cooper David ; Sumner, Esther J. ; Tank, Suzanne E. ; Talling, Peter J.
    The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may lower the concentrations of atmospheric CO2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river-fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within Bute Inlet, a fjord in western Canada. We show that 62% ± 10% of the OC supplied by the two river sources is buried across the fjord surficial (30–200 cm) sediment. The sandy subenvironments (channel and lobe) contain 63% ± 14% of the annual terrestrial OC burial in the fjord. In contrast, the muddy subenvironments (overbank and distal basin) contain the remaining 37% ± 14%. OC in the channel, lobe, and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord’s surface area, at least 3 times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long-term (>100 years) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient at storing OC supplied by rivers in their near-surface deposits.
  • Article
    Efficient preservation of young terrestrial organic carbon in sandy turbidity-current deposits
    (Hage, S., Galy, V. V., Cartigny, M. J. B., Acikalin, S., Clare, M. A., Grocke, D. R., Hilton, R. G., Hunt, J. E., Lintern, D. G., McGhee, C. A., Parsons, D. R., Stacey, C. D., Sumner, E. J., & Talling, P. J. (2020). Efficient preservation of young terrestrial organic carbon in sandy turbidity-current deposits. Geology, 48(9), 882-887., 2020-05-29) Hage, Sophie ; Galy, Valier ; Cartigny, Matthieu J. B. ; Acikalin, Sanem ; Clare‬, Michael A. ; Grocke, Darren R. ; Hilton, Robert G. ; Hunt, James E. ; Lintern, D. Gwyn ; McGhee, Claire A. ; Parsons, Daniel R. ; Stacey, Cooper David ; Sumner, Esther J. ; Talling, Peter J.
    Burial of terrestrial biospheric particulate organic carbon in marine sediments removes CO2 from the atmosphere, regulating climate over geologic time scales. Rivers deliver terrestrial organic carbon to the sea, while turbidity currents transport river sediment further offshore. Previous studies have suggested that most organic carbon resides in muddy marine sediment. However, turbidity currents can carry a significant component of coarser sediment, which is commonly assumed to be organic carbon poor. Here, using data from a Canadian fjord, we show that young woody debris can be rapidly buried in sandy layers of turbidity current deposits (turbidites). These layers have organic carbon contents 10× higher than the overlying mud layer, and overall, woody debris makes up >70% of the organic carbon preserved in the deposits. Burial of woody debris in sands overlain by mud caps reduces their exposure to oxygen, increasing organic carbon burial efficiency. Sandy turbidity current channels are common in fjords and the deep sea; hence we suggest that previous global organic carbon burial budgets may have been underestimated.