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1. Synchronous Marine and Terrestrial Carbon Cycle Perturbation in the High Arctic During the PETM

   https://doi.org/10.1029/2020PA003942  

   Cui, Ying; Diefendorf, Aaron_F; Kump, Lee_R; Jiang, Shijun; Freeman, Katherine_H (April 2021, Paleoceanography and Paleoclimatology)

   Abstract The Paleocene‐Eocene Thermal Maximum (PETM; 56 Ma) is considered to be one of the best analogs for future climate change. The carbon isotope composition (δ¹³C) ofn‐alkanes derived from leaf waxes of terrestrial plants and marine algae can provide important insights into the carbon cycle perturbation during the PETM. Here, we present new organic geochemical data and compound‐specific δ¹³C data from sediments recovered from an early Cenozoic basin‐margin succession from Spitsbergen. These samples represent one of the most expanded PETM sites and provide new insights into the high Arctic response to the PETM. Our results reveal a synchronous ∼−6.5‰ carbon isotope excursion (CIE) in short‐chainn‐alkanes (nC₁₉; marine algae/bacteria) with a ∼−5‰ CIE in long‐chainn‐alkanes (nC₂₉andnC₃₁; plant waxes) during the peak of the PETM. Although δ¹³C_n‐alkanesvalues were potentially affected via a modest thermal effect (1‰–2‰), the relative changes in the δ¹³C_n‐alkanesremain robust. A simple carbon cycle modeling suggests peak carbon emission rate could be ∼3 times faster than previously suggested using δ¹³C_TOCrecords. The CIE magnitude of both δ¹³C_n‐C19and δ¹³C_n‐C29can be explained by the elevated influence of¹³C‐depleted respired CO₂in the water column and increased water availability on land, elevatedpCO₂in the atmosphere, and changes in vegetation type during the PETM. The synchronous decline in δ¹³C of both leaf waxes and marine algae/bacteria argues against a significant contribution to the sedimentary organic carbon pool from the weathering delivery of fossiln‐alkanes in the Arctic region. 

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2. Ongoing Increase in Eastern Tropical North Pacific Denitrification as Interpreted Through the Santa Barbara Basin Sedimentary δ ¹⁵ N Record

   https://doi.org/10.1029/2019PA003578  

   Davis, C. V.; Ontiveros‐Cuadras, J. F.; Benitez‐Nelson, C.; Schmittner, A.; Tappa, E. J.; Osborne, E.; Thunell, R. C. (September 2019, Paleoceanography and Paleoclimatology)

   Abstract Decades of observations show that the world's oceans have been losing oxygen, with far‐reaching consequences for ecosystems and biogeochemical cycling. To reconstruct oxygenation beyond the limited scope of instrumental records, proxy records are needed, such as sedimentary δ¹⁵N. We combine two δ¹⁵N records from the Santa Barbara Basin (SBB), a 24‐year‐long, biweekly sediment trap time series, and a 114‐year, high‐resolution sediment core together spanning the years 1892–2017. These records allow for the examination of δ¹⁵N variability on seasonal to centennial timescales. Seasonal variability in SBB δ¹⁵N is consistent in timing with the poleward advection of a high δ¹⁵N signal from the Eastern Tropical North Pacific in the summer and fall. Strong El Niño events result in variable δ¹⁵N signatures, reflective of local rainfall, and neither the Pacific Decadal Oscillation nor North Pacific Gyre Oscillation impose strong controls on bulk sedimentary δ¹⁵N. Seasonal and interannual variability in sediment trap δ¹³C_orgis consistent with local productivity as a driver; however, this signal is not retained in the sediment core. The time series from the sediment trap and core show that bulk sedimentary δ¹⁵N in SBB has now exceeded that measured for the past 2,000 years. We hypothesize that the change in δ¹⁵N reflects the increasing influence of denitrified waters from the Eastern Tropical North Pacific and ongoing deoxygenation of the Eastern Pacific. When juxtaposed with other regional δ¹⁵N records our results further suggest that SBB is uniquely situated to record long‐term change in the Eastern Tropical North Pacific. 

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3. Efficient preservation of young terrestrial organic carbon in sandy turbidity-current deposits

   https://doi.org/10.1130/G47320.1  

   Hage, S.; Galy, V.V.; Cartigny, M.J.B.; Acikalin, S.; Clare, M.A.; Gröcke, D.R.; Hilton, R.G.; Hunt, J.E.; Lintern, D.G.; McGhee, C.A.; et al (May 2020, Geology)

   null (Ed.)

   Abstract 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. 

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4. Transport and distribution of terrestrial plant biomarkers in modern lake sediments: implications for paleoenvironmental reconstructions in the Adirondack Mountains

   Bates, B.R.; Lowell, T.V.; Diefendorf, A.F.; Freimuth, E.J.; Stewart, A.K. (September 2017, 6th annual Midwestern Geobiology Conference)

   Terrestrial plant biomarkers preserved in lake sediments are commonly used in paleoenvironmental reconstructions. Basin-specific transport pathways and distribution controls of plant biomarkers, however, are poorly understood. This study mapped the distribution of sedimentary n-alkanes sourced from vascular plant waxes to delineate possible transport pathways and quantified the contribution of terrestrial and aquatic input. We combine these data with existing leaf and pollen taphonomy literature and sediment focusing models to develop a better understanding of the controls on plant biomarker transport within lake basins. Here, we report the spatial distribution of sedimentary n-alkanes, the carbon isotope values and C:N ratios of bulk sediment, and percent organic matter from three lakes in the Adirondack Mountains, NY. Preliminary carbon isotope data and n-alkane concentrations within each lake suggests a large terrestrial input. Bulk sediment carbon isotope values ranged from - 26‰ to -32‰ consistent with carbon isotope values of modern terrestrial vegetation. The concentrations of long-chain n-alkanes (indicative of higher land plants), moreover, are much higher than short-chain n-alkanes (indicative of aquatic and microbial activity) by almost two times. By contrast, C:N ratios range from 11-14 indicating a mix of aquatic and terrestrial contribution to the lake’s total organic matter. We combined high-resolution sonar data with the sediment analyses to identify basin- specific controls on the distributions of n-alkanes and bulk sediment carbon isotopes. The statistical categorization of sediment zones based on relative hardness and roughness along the lake bottom delineates where organic material is concentrated. For the terrestrially sourced plant waxes, we measured low n-alkane concentrations in sandy littoral sediments relative to deeper sediments towards the main depo-center. Together, this information validates sediment focusing models and suggests that terrestrial carbon and n-alkanes are preferentially transported to the main depo-center of the lake. These observations highlight important relationships between basin-specific sediment properties and processes controlling the transport and deposition of n- alkanes. 

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5. Influence of the Keystone Grazer, Sesarma reticulatum, on the Hydrology and Organic Matter Cycling in Salt Marshes of the Southeastern USA

   https://doi.org/10.1007/s12237-024-01336-9  

   Morrison, Elise S.; Bianchi, Thomas S.; Kenney, William F.; Brenner, Mark; Prince, Kimberly; Williams, Sydney; Ortals, Collin; Cordero, Orlando; Crotty, Sinéad M.; Angelini, Christine (March 2024, Estuaries and Coasts)

   Abstract In salt marshes of the Southeastern USA, purple marsh crabs (Sesarma reticulatum), hereafterSesarma, aggregate in grazing and burrowing fronts at the heads of tidal creeks, accelerating creek incision into marsh platforms. We explored the effects of this keystone grazer and sediment engineer on salt marsh sediment accumulation, hydrology, and carbon (C) and nitrogen (N) turnover using radionuclides (²¹⁰Pb and⁷Be), total hydrolyzable amino acids (THAA), and C and N stable isotopes (δ¹³C and δ¹⁵N) in sediment from pairedSesarma-grazed and un-grazed creeks.Sesarma-grazed-creek sediments exhibited greater bioturbation and tidal inundation compared to sediments in un-grazed creeks, as indicated by larger²¹⁰Pb and⁷Be inventories. Total organic carbon (TOC) to total nitrogen (TN) weight ratios (C:N) were higher and δ¹⁵N values were lower in grazed-creek sediments than in un-grazed-creek sediments, suggestingSesarmaremove and assimilate N in their tissues, and excrete N with lower δ¹⁵N values into sediments. In support of this inference, the percent total carbon (TC) and percent TOC declined by nearly half, percent TN decreased by ~ 80%, and the C:N ratio exhibited a ~ threefold increase betweenSesarmafore-gut and hind-gut contents. An estimated 91% ofSesarma’s diet was derived fromSpartina alterniflora,the region’s dominant salt marsh plant. We found that, asSesarmagrazing fronts progress across marsh landscapes, they enhance the decay ofSpartina-derived organic matter and prolong marsh tidal inundation. These findings highlight the need to better account for the effects of keystone grazers and sediment engineers, likeSesarma, in estimates of the stability and size of blue C stores in coastal wetlands. 

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